The present invention relates to ball valves of known type comprising a housing having an interior cavity located between and communicating with a pair of fluid flow channels to define an axial flow passage extending through the housing, a ball located in the interior cavity, the ball having an axially-directed bore extending therethrough and being a floating ball that is rotatable through substantially 90.degree. about an axis of rotation transverse to the flow passage whereby the bore may be selectively aligned with or disposed transverse to the axial flow passage in the housing to control the flow of fluid therethrough. The ball cooperates with a pair of seats that are supported by the housing adjacent the upstream and downstream sides of the cavity, respectively, for sealing engagement with the housing and ball. When the seats are round hole seats, such valves are "high recovery" valves, i.e., most of the fluid pressure available upstream of the valve is still available downstream of the valve after the media, i.e., fluid passes through the valve.
Ball valves of the types described above are inherently low loss devices. They exhibit excellent performance for on/off service, but do not exhibit desirable characteristics for control service. In control service, the ball position is changed, i.e., modulated to vary the flow through the valve or to vary the pressure downstream of the valve. High recovery valves are increasingly being used for such service because their 90.degree. operation makes automation simple. The control that can be achieved by such valves, however, is inferior to traditional rising stem control valves in three areas, i.e., rangeability, flow characteristic and pressure recovery. The term "flow characteristic" is defined as the relation between flow through the valve and percent rated travel as the valve position is varied from 0 to 100%. The term "inherent flow characteristic", with which the present invention is primarily concerned, means the flow characteristic of the valve when constant pressure drop is maintained across the valve. The inherent flow characteristic can comprise an "equal percentage flow characteristic", i.e., an inherent flow characteristic which, for equal increments of rated travel, will ideally give equal percentage changes of the existing flow. The flow characteristic may also be a "linear flow characteristic", i.e., an inherent flow characteristic which can be represented ideally by a straight line on a rectangular plot of flow versus percent rated travel so the equal increments of travel yield equal increments of flow at a constant pressure drop. A round hole seat cooperating with a round hole bore in a valve ball typically achieves an equal percentage flow characteristic. The flow characteristic may also be "quick opening", i.e., the flow characteristic achieved by a V-shaped opening in the seat with the large end of the V exposed first as the ball moves toward full open position.
Rangeability is the ratio of highest to lowest controllable flow rate. The term "inherent rangeability" is defined as the ratio of maximum to minimum flow within which the deviation from the specified inherent flow characteristic does not exceed a stated limit, usually 1%. The inherent rangeability of a round hole valve can be 30 or 50 to 1, while the rangeability of a ball valve using a V-seat or a slotted seat can be 100 to 200 to 1. The advantage of using a V-seat or slotted seat is better control at both ends of the range, allowing one valve to be used instead of requiring the use of two or more valves in services having wide requirements. Moreover, valves using slotted seats provide, and valves using V-seats approach, a linear flow characteristic. With all other valve designs, inherent rangeability is rarely realized in practice. This is due to the normal procedure of selecting a valve flow coefficient (C.sub.v) which exceeds that which is actually needed. This greater C.sub.v is specified because of the concern of avoiding undersizing of a valve, which can easily happen because flow and pressure data are often inaccurate, or they change due to the need to alter the process.
Other designs have limited, stepwise C.sub.v changes available, and it is only through happenstance that an exact match occurs between the required and the available C.sub.v.
The valve design of the present invention overcomes these problems because there are an infinite number of C.sub.v values available between the maximum possible with a fully open, round port valve and the minimum possible with a highly restricted valve using the present invention. Therefore, the need to oversize a valve is no longer necessary because of the ease with which a C.sub.v change can be made, and because of the possibility of exactly matching the requirement. This represents a significant and drastic change in valve selection and sizing philosophy, and results in significant improvements in the accuracy and performance of control systems. The valve is the one element of control systems which has been ignored with respect to process accuracy, whereas all other elements (sensors, controllers, wiring, communications) have been exhaustively improved in order to gain higher accuracy.
Another element of the design concept embodied in the present invention relates to its contribution to the dynamic performance of the system in which it is used. As described above, it is normal to specify oversized valves; however, this is highly undesirable because oversizing makes a control loop more sensitive, since a fixed change in valve position results in a greater change in flow or pressure than would be desirable if a valve were used with the exactly-needed C.sub.V. This greater-than-desired change causes excessive process sensitivity, the process "overshoots", and must be corrected by adjusting the sensitivity of another element of the control system. When this procedure, called tuning, is performed, the system becomes desensitized and the result is a process which responds slowly to changes. This slow response means that assets such as energy or raw materials are wasted. With the present invention, industry is able to make the final improvement in piping processes which will enable it to maximize performance of control loops.
Pressure recovery involves considerations of flashing, cavitation and excess noise. In the absence of pressure reduction across the valve, it is not possible to modulate or vary the flow through the valve, i.e., to control effectively and to achieve a reduction in flow, the valve must burn up pressure or energy. High recovery valves use fluid to fluid contact, i.e., turbulence, to reduce pressure, which results in cavitation, etc. A much better way to reduce the pressure recovery capability of a valve is to increase the fluid to metal contact in the valve, i.e., to increase the friction.
A ball valve which relies upon this latter concept to achieve pressure reduction in a ball valve is disclosed in Baumann U.S. Pat. No. 4,085,774. The Baumann valve includes a plate, secured in the outlet flow passage of a ball valve, that is provided with a plurality of anti-cavitation passageways which effect a high loss of kinetic energy and which reduce noise by subdividing the fluid flow through the outlet flow passage of the valve into a large number of small streams. However, the anticavitation plate in the Baumann valve is not in sealing contact with the rotatable ball of the valve; instead, the rotatable ball in the Baumann valve is of the trunnion type rather than being a floating ball, and is associated on the downstream side of the valve with a thin and flexible metal seat ring that is in sealing engagement with the ball in spaced relation to the upstream surface of the anticavitation plate to avoid excessive wear and high operating torque. Because of the space that is necessarily provided by this arrangement between the Baumann ball and anticavitation plate, when the ball is in other than the fully closed valve position, fluid leakage occurs through those passageways in the anticavitation plate which are ostensibly covered by the valve ball. As a result, there is an increase in the minimum flow which occurs through the valve when the Baumann ball is in other than its fully closed position, with a consequent significant decrease in the inherent rangeability of the valve. Since rangeability and precise control are related factors, the Baumann valve, due to said leakage, does not provide very precise control at the low end of the rangeability ratio.
The present invention is intended to obviate problems of the types discussed above with respect to the Baumann valve arrangement.